Overvoltage arrester

ABSTRACT

An overvoltage arrester has a stack of spark gap assemblies, each having two disc-shaped metallic electrodes and a weakly conductive spacer member disposed between the two electrodes and forming a current limiting resistor in addition to the spacer function. Each of the spacer members is constituted by a ring which has a generally Z-shaped cross-section thus forming a step configuration of an inner ring portion integrally joined with an outer portion of larger diameter. The outer portion surrounds the inner portion of the next adjacent ring in the stack, the spacer member proper being constituted substantially only by the inner portion of the ring.

United States Patent Jakszt et a1. May 23, 1972 54] OVERVOLTAGE ARRESTER 3,152,279 10/1964 Misare .ms/sx [72] memos: Werner Jam; Gen Schick; Wolfgang PrimaryExaminer-James D. Trammell Werner, all of Berlin, Germany [73] Assignee: Siemens Aktiengesellschaft, Berlin,

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[22] Filed: Sept. 4, 1970 [21] App1.No.: 69,624

[30] Foreign Application Priority Data Sept. 9, 1969 Germany ..P 19 46 416.5

[52] US. Cl.... ...317/68, 315/36, 317/70 [51 1 int. Cl. ..H02h 9/06 [58] Field of Search ..3 17/61, 68, 70; 315/36;

3 l 3/DIG. 5

[56] References Cited UNITED STATES PATENTS 3,366,832 l/ 1268 Sarbach ..3 151,36

Attorney-Curt M. Avery, Arthur E. Wilfond, Herbert L. Lerner and Daniel .1. Tick ABSTRACT An overvoltage arrester has a stack of spark gap assemblies, each having two disc-shaped metallic electrodes and a weakly conductive spacer member disposed between the two electrodes and forming a current limiting resistor in addition to the spacer function. Each of the spacer members is constituted by a ring which has a generally Z-shaped cross-section thus forming a step configuration of an inner ring portion integrally joined with an outer portion of larger diameter. The outer portion surrounds the inner portion of the next adjacent ring in the stack, the spacer member proper being constituted substantially only by the inner portion of the ring.

3 Claims, 3 Drawing Figures Patented May 23, 1972 3,665,255

2 Sheets-Sheet 1 Patented May 23, 1972 3,665,255

2 Sheets-Sheet 2 OVERVOLTAGE ARRESTER The invention relates to an overvoltage arrester of the type having a stack of several spark gaps of which each is formed by two disc-shaped metallic electrodes which are spaced from each other by a weakly conductive spacer piece simultaneously functioning as a current limiting or control resistor.

It is an object of the invention to improve such overvoltage arresters which are increasing the protection of the spark gap stack in electrical respects as well as in mechanical respects.

Another object of the invention is to afford or facilitate adapting the cross-sections of the arrester components to the most favorable electrical data.

Still another object of the invention, subsidiary to those just mentioned is to afford the desired improvements in a particularly simple manner readily lending itself to mass production with the aid of automatic machinery.

To achieve these objects and in accordance with the invention, the above-mentioned spacer piece is constituted by the inner annular portion of the ring whose cross-section has substantially the shape of a Z, the inner portion of the ring being surrounded by and preferably nested in, the ring outer portion of the next adjacent ring of the stack.

More specifically, in an overvoltage arrester according to the invention, the rings of high resistivity material have a stepshaped cross-section, whereby the above-mentioned inner ring portion and outer portion are formed. The rings adjacent in the stack are coaxially nested in each other, the electrodes being constituted by metal discs individually interposed between the bottom face of an inner ring portion and the outer face of the next adjacent ring.

In such an arrester, the ring inner portion of each spacer can be looked upon as constituting the control or current limiting resistor proper. That is, the inner portion of low conductivity resistance material influences the voltage within the electrodes substantially independently of the ring outer portion, for example, by restricting the leakage-discharging of any undesired capacitive charges between the electrodes. The outer portion of the ring, however, is effective as a shield which keeps disturbing influences away from the spark gap, particularly such spurious effects as may be due to the occurrence of foreign deposits upon the arrester housing. In addition, the ring outer portion may be utilized as mechanical protection of the ring inner portion operating as charge-limiting resistor between the electrodes. As a consequence, it is not necessary to take any mechanical strength values into account with respect to the dimensions of the ring inner portion so that the cross-section of shapes and areas can be chosen in accordance with the most desired electrical properties.

It is desirable in arresters according to the invention that the outer portion as well as the inner portion is electrically connectable at the respective axial end faces. For this reason, and in accordance with a further feature of the invention, the axial height of the outer portion is made a bit larger than the height of the inner portion, the difference being equal to the thickness of an electrode disc. With such a dimensioning the outer portion and the inner portion of the ring provide a uniform contact with the electrode disc when the rings are nested into each other to form the spark gap stack. If desired, however, metallic spring elements or other elastic and electrical conducting inserts may be used for equalizing the differences in height so that the height of the respective outer ring portions can be dimensioned,within limits, independently of each other. It is further preferable to provide a conducting coating upon the contact faces of the ring, that is the faces engageable with the electrode discs, thus preventing discharges.

Tests have shown that it is advisable in some cases to give the top face of the ring inner portion a bevelled shape at the inner circular region so that the spacer member and the adjacent electrode disc define an acute-angled gusset. As is known, pre-discharges will occur in the gusset areas. These pre-discharges cause an ionization of the spark gap enclosed within the stack of spacer members, so that the discharge delay is reduced in the desired manner.

The invention will be further described with reference to embodiments illustrated by way of example on the accompanying drawings in which:

FIG. 1 is a sectional view of a complete overvoltage arrester of the invention;

FIG. 2 shows in cross-section a proportion of the spark gap assembly which is part of the arrester according to FIG. 1; and

FIG. 3 is a partial sectional view of a modified form of a spacer and electrode assembly, this embodiment being otherwise in accordance with FIGS. 1 and 2.

Referring to FIG. 1, there is illustrated an overvoltage arrester, for example, rated for a voltage of 20 kW. The arrester comprises a tubular housing of porcelain closed by metal armatures 2 and 3 fixedly cemented to the housing. A clamp 4 for installing the arrester, as well as the terminal bolt of the bottom armature 3 is conventionally connected to ground, the high voltage being applied to the terminal bolt integral with the top armature 2.

The interior space of the tubular housing 5 is filled with the active elements of the arrester. These comprise series connected resistance discs 7 preferably consisting of silicon-carbide (SiC) and the quenching spark gap 8 that determines the critical minimum (threshold) voltage at'which the arrester is to respond, the spark gap 8 having further the function of interrupting the occurring electric current which is driven through the arrester by the effect of the line voltage after the arrester has commenced to respond.

The quenching spark gap 8 is a multiple gap consisting of electrode discs which are stacked upon each other and are kept spaced from each other by intermediate spacer members. As will be seen from FIG. 2, the spark gap assembly comprises a number of coaxially stacked rings 10 which have a generally C-shaped cross-section, that is the cross-section of each ring 10 forms a step and has two substantially equal rectangular portions 11 and 12 which merge with each other at 13. In the embodiment herein described, the ring portions 11 and 12 consist of the same material, for example of ceramic material or of synthetic plastic material such as an epoxy resin. This material, being an insulator, is made weakly conducting by an addition of 20 percent by weight of silicon carbide. The electrical resistivity of the spacer ring material is preferably 10 Ohm-cm. The invention, however, can also be realized by using materials of respectively difi'erent conductivity for the two ring portions 11 and 12. The different materials may be integrally joined with each other in a suitable manner, preferably by a thermal process such as welding or sintering.

The electrodes in the illustrated embodiment are made as planar discs 16 which have the same shape and size. The electrodes are made preferably of a special alloy such as a brass alloy available for such electrical purposes, each electrode disc has a projection 17 at only one side which is preferably stamped and bent out of the electrode foil material. The projection 17, together with the planar face of the adjacent electrode forms an ignition location 18 at which the distance between the electrodes is a minimum. On account of this minimum distance, the occurrence of an overvoltage will cause a breakdown to be initiated at this per-determined locality so that an arc will be produced. The projection 17 is excentrically arranged. As a consequence, the arc is placed in motion by the pressure wave of the gas oscillating in the arrester chamber. The foot point of the arc travels from the projection 17 on to the plane of the electrode disc. This holds the local amount of burning at the ignition locality 18 to small limits.

As will be seen from FIG. 2, the rings 10 of the stack are nested in each other so that the ring inner portion 12 of each spacer member is coaxially surrounded by the outer portion of the adjacent ring. Since the height of the ring outer portion 11 is larger by the thickness of an electrode disc 16 than the height of the inner portion 12, the inner portions 11 and 12 extending parallel to each other, are electrically connected in series. The ring inner portions therefore constitute control or limit or grading resistors which are connected in parallel relation to the spark gaps 18 of the electrode disc 16, and which discharge any capacitive charges that otherwise might impair the characteristic of response in the event of rapidly repetitive unipolar voltage surges.

The series connected outer portions 11 of the rings 10, which are in parallel relation to the stack of electrodes, function as a shielding of the electrodes against voltage stresses that might be imposed from the outside upon the spark gap 8. This shielding of the spark gap provides a uniform threshold voltage at which the arrester will respond even in the event of non-uniform soiling of the arrester housing 1 which otherwise is apt to result in non-uniform voltage distribution at the outer surface of the housing. In addition, a mechanical strengthening is achieved by virtue of the fact that the ring inner portions 12 are closely surrounded by the outer portions 11 of the adjacent rings.

According to FIG. 3, the cross-section of the ring inner portion 12 is advantageously modified to a relatively slight extent with respect to mechanical shape and production, that is in the embodiment shown in FIG. 3, the inner edge 22 of portion 12 is given a bevel 20 at the top face 23. Together with the electrode disc 16, this bevel defines a triangular space or gusset 21 in which pre-discharges will occur. Seen in cross-section according to FIG. 3, the bevel at 20 extends at an acute angle with respect to the planar face of the electrode disc 16. Due to the pre-discharges, an ionization occurs in the space between the electrode discs when the voltage increases close to or beyond the critical value; and such ionization advantageously shortens the delay in response, otherwise possible.

Upon a study of this disclosure, it will be obvious to those skilled in the art that arresters'according to the invention may be modified in various respects and may be given embodiments other than those particularly illustrated and described herein.

We claim:

1. In an overvoltage arrester comprising a stack of spark gap assemblies each having two disc-shaped metallic electrodes and a spacer member made of weakly conductive material, the spacer member being disposed between said two electrodes and fonning a grading resistor, the improvements according to which each of said spacer members is an annular ring having a step-shaped cross-section, said ring having an inner portion and an outer ring portion of larger diameter than said inner ring portion, said spacers being coaxially stacked one inside the other so that said outer portion surrounds the inner portion of the next adjacent spacer in said stack, said electrode discs being individually interposed between the bottom face of an inner ring portion and the top face of the inner ring portion of the next adjacent ring so as to be in electrical contact with said faces, and said outer ring portion having an axial length larger, by the thickness of one of said electrode discs, than that of said inner ring portion.

2. In an overvoltage arrester according to claim 1, said ring portions having a conductive coating on the electrode-contacting faces.

3. In an overvoltage arrester according to claim 1, said top face and said bottom face of each of said inner ring portions having at the inner edge a bevel at an acute angle to the adjacent one of said electrode discs. 

1. In an overvoltage arrester comprising a stack of spark gap assemblies each having two disc-shaped metallic electrodes and a spacer member made Of weakly conductive material, the spacer member being disposed between said two electrodes and forming a grading resistor, the improvements according to which each of said spacer members is an annular ring having a step-shaped cross-section, said ring having an inner portion and an outer ring portion of larger diameter than said inner ring portion, said spacers being coaxially stacked one inside the other so that said outer portion surrounds the inner portion of the next adjacent spacer in said stack, said electrode discs being individually interposed between the bottom face of an inner ring portion and the top face of the inner ring portion of the next adjacent ring so as to be in electrical contact with said faces, and said outer ring portion having an axial length larger, by the thickness of one of said electrode discs, than that of said inner ring portion.
 2. In an overvoltage arrester according to claim 1, said ring portions having a conductive coating on the electrode-contacting faces.
 3. In an overvoltage arrester according to claim 1, said top face and said bottom face of each of said inner ring portions having at the inner edge a bevel at an acute angle to the adjacent one of said electrode discs. 